Valley Exciton Modulation in Janus MoSeS via Phase‐Engineered Intrinsic Polarization Fields

ABSTRACT Janus transition metal dichalcogenides host a unique structural asymmetry that enhances their potential for spintronics and valleytronics. Here, using a phase engineering strategy, a hybrid H/T‐MoSeS monolayer is synthesized via in situ plasma substitution. Theoretical analysis predicts a valley splitting of 7.35 meV, which is experimentally realized, with a 2.7‐fold enhancement in the g‐factor compared to the H‐MoSeS. The substantial valley splitting originates from the additional exchange interactions and spin imbalance introduced by the T‐phase. The coupling with the T phase further results in electrically tunable valley polarization, which reveals a sharp increase within ±20 V, gradually saturates at higher voltages, and reaches a high ratio of up to 43.8% for trions at room temperature. Such an enhancement is accompanied by elongated exciton lifetimes due to the increased electron concentration and phase‐modulated Janus polarization field. All these findings reveal the synergistic interplay between phase engineering and the polarization field in Janus structures, providing a development strategy for 2D spin‐valley devices with designed phase components and structural symmetry.